i.l THE QUEST OF A CRITERION 55 



would be proportional both to the divergency between the 

 lines of evolution thus chosen and to the complexity of the 

 similar structures found in them. 



It will be said that resemblance of structure is due to 

 sameness of the general conditions in which life has evolved, 

 and that these permanent outer conditions may have 

 imposed the same direction on the forces constructing 

 this or that apparatus, in spite of the diversity of transient 

 outer influences and accidental inner changes. We are 

 not, of course, blind to the role which the concept of 

 adaptation plays in the science of to-day. Biologists cer- 

 tainly do not all make the same use of it. Some think 

 the outer conditions capable of causing change in organ- 

 isms in a direct manner, in a definite direction, through 

 physico-chemical alterations induced by them in the liv- 

 ing substance; such is the hypothesis of Eimer, for example. 

 Others, more faithful to the spirit of Darwinism, believe 

 the influence of conditions works indirectly only, through 

 favoring, in the struggle for life, those representatives of a 

 species which the chance of birth has best adapted to the 

 environment. In other words, some attribute a positive 

 influence to outer conditions, and say that they actually 

 give rise to variations, while the others say these conditions 

 have only a negative influence and merely eliminate varia- 

 tions. But, in both cases, the outer conditions are sup- 

 posed to bring about a precise adjustment of the organism 

 to its circumstances. Both parties, then, will attempt 

 to explain mechanically, by adaptation to similar condi- 

 tions, the similarities of structure which we think are the 

 strongest argument against mechanism. So we must at 

 once indicate in a general way, before passing to the detail, 

 why explanations from "adaptation" seem to us insufficient. 



Let us first remark that, of the two hypotheses just 

 described, the latter is the only one which is not equivocal. 



